The present invention relates to an improved line pressure control device of an automatic transmission for vehicle.
In an automatic transmission line pressure for a actuating oil from an oil pump driven by the engine is usually controlled to be a predetermined oil pressure by a line pressure adjusting valve. Manifold negative pressure from the engine is introduced into a vacuum diaphragm of a throttle valve to generate a throttle pressure corresponding to the engine load. A governor pressure corresponding to a vehicle speed is generated from the line pressure through a governor secured to an output shaft of the automatic transmission. A hydraulic pressure change-over valve is activated (shift valve) by the throttle pressure and the governor pressure and, properly supplies the line pressure to friction elements such as a clutch, a brake and the like for changing over a power transmission route of a pair of planetary gears, and performs automatic transmission operation. The throttle pressure is also introduced into the line pressure adjusting valve so as to adjust the line pressure corresponding to the load of the engine, so that the friction elements actuated by the line pressure do not produce any slippage.
Exahust gas from automobiles is now strictly regulated and a countermeasure to control exhaust gas to meet standards from each car is necessary. Under the influence of, for example, an exhaust reflux device for returning a part of exhaust gas to an engine inlet system, the engine manifold negative pressure does not precisely correspond to the load of the engine. In the above-described prior automatic transmission, therefore, the line pressure adjusted by the manifold negative pressure will not correspond to the load of the engine and the line pressure becomes too great or too little compared to the load of the engine. As a result, the clamping force of the friction element actuated by the line pressure becomes too large producing a transmission shock or the clamping or coupling force is too weak making the friction element slip. Because of these disadvantages the engine output is not effectively utilized, fuel expense becomes greater and the friction element is damaged.
It has been proposed in an automatic transmission not to directly introduce the manifold negative pressure into the vacuum diaphragm of the throttle valve controlling the line pressure. Instead a negative pressure corresponding to the degree of throttle opening (accelerator pedal depression amount) representing the load of the engine is produced, and the negative pressure is introduced into the vacuum diaphragm for the purpose of avoiding the above described disadvantages.
An example of such an automatic transmission with an electronic control is shown in FIG. 1. In the system of FIG. 1, the throttle opening degree, manifold negative pressure, inlet air amount or fuel injection amount and the like are detected. A signal corresponding to the load of the engine is supplied from engine load sensor 20 and a signal corresponding to a car or vehicle speed is supplied from a vehicle speed sensor 21 to a shift control unit 22, respectively, and the shift control unit 22 properly actuates the shift valves 23 in response to these input signals. An oil pump 24 driven by the engine sucks and exhausts actuating oil from an oil reservoir 25. The actuating oil is adjusted to a line pressure by a line pressure adjusting valve 26 and is constantly supplied to a manual valve 27. When a driver operates the manual valve 27 to an operating range, the line pressure is introduced into a shift valve selected from the shift valves 23, and the shift control unit 22 selects and actuates the shift valves 23 in accordance with the driving state of the automobile, so that the line pressure is supplied to one of the friction elements, 28, actuates this friction element and carries out automatic transmission operation.
The shift control unit 22 applies the engine load signal to a hydraulic pressure adjusting valve (vacuum throttle valve) 29. The hydraulic pressure adjusting valve 29 produces a hydraulic pressure corresponding to the engine load acting on the line pressure adjusting valve 26 in accordance with the engine load signal, operates on the line pressure adjusting valve 26 and functions to make the line pressure precisely correspond to the magnitude of the engine load. A line pressure control system is represented in FIG. 2 in more detail. The block 29 of FIG. 1 identified by the legend hydraulic pressure adjusting valve includes a vacuum diaphragm 2, a negative pressure conduit 3, a diaphragm device 4, a negative pressure solenoid valve 9 and an atmospheric pressure solenoid valve 10 in addition to the hydraulic pressure adjusting valve provided in the automatic transmission body 1. An electronic circuit 8 (FIG. 2) is provided within the shift control unit 22 shown in FIG. 1. The vacuum diaphragm 2 receives a negative pressure in the negative pressure chamber 4A of the diaphragm device 4 through the conduit 3. This negative pressure actuates the hydraulic pressure ajdusting valve through the vacuum diaphragm 2 and adjusts the line pressure as described above, so that the magnitude of the negative pressure (absolute value) corresponds to the line pressure in inverse proportion. The negative pressure in the chamber 4A is electrically detected as a line pressure signal by means of a line pressure sensor operated in response to the displacement of the diaphragm device 4 and comprising a variable resistor or the like. The amount of depression pedal 6 of the vehicle accelerator corresponding to the engine load, is electrically detected by a throttle sensor 7 comprising a variable resistor or the like and the throttle sensor is used as an engine load sensor. An actually measured line pressure signal V.sub.q and an engine load signal V.sub.T from the line pressure sensor 5 and the throttle sensor 7 vary, respectively, as shown in FIGS. 3 and 4. These signals are supplied to the electronic circuit 8. The electronic circuit 8 compares the signal V.sub.q representing the line pressure with the signal V.sub.T representing the engine load and determines whether the signal V.sub.q corresponds to the ideal line pressure as shown by the graph in FIG. 5. When the line pressure uses above the ideal value (i.e., the actually measured line pressure signal V.sub.q is larger than a target negative pressure V.sub.T '), the negative pressure solenoid valve 9 is opened by supplying a signal thereto. A vacuum tank 12 storing the negative pressure from the engine manifold through a check valve 11 will then supplement the negative pressure to the vacuum diaphragm 2 so as to actuate the hydraulic pressure adjusting valve to decrease the line pressure. In the opposite case, the atmospheric pressure solenoid valve 10 is opened by supplying a signal thereto and the hydraulic pressure adjusting valve is actuated to increase the line pressure. As a result, the line pressure is controlled along target characteristics shown in FIG. 5 to correspond to the engine load, and even if the manifold negative pressure does not precisely correspond to the engine load due to the exhaust gas countermeasure, the automatic transmission can be controlled as desired.
In such an automatic transmission, however, when a diaphragm 4B of the diaphragm 4 does not promptly stop at a balanced position between the negative pressure and a spring 4C during the above described operation, but over shoots, the operational stability of the control system suffers the amount of power consumption by the solenoids valve 9, 10 is increased and the amount of negative pressure consumption in the vacuum tank 12 is increased.
In order to solve this problem, stabilizing the control system by providing a dead zone for inactivating both the solenoids 9 and 10 has been considered, but this kind of conventional countermeasure only provides a dead zone on the + side or - side to the target value, so that the over shoot in the vicinity of the target value is prevented in one direction but not in the opposite direction. As a result, the problem is not always avoided and the actual target value is not achieved. Moreover, it has hitherto been considered to provide a pulse width for determining the opening time of the solenoids 9, 10 in accordance with the magnitude of deviation of the actually measured value from the target value, thereby improving the response of the control system and promptly positioning the actually measured value in the dead zone. However, in this case, the dead zone is set only on one side of the target value as described above and being the deviation is measured with the target value being a boundary value of the dead zone. For this reason, controlling the pulse width in accordance with deviation for the actually measured value in does not tend to promptly position the dead zone in the strict sense, and these countermeasures do not always attain the desired object.